Maintenance feature in magnetic implant

ABSTRACT

A distraction system includes a distraction rod having one end configured for affixation to at a first location on patient. The system further includes an adjustable portion configured for placement in the patient at a second location, the adjustable portion comprising a housing containing a magnetic assembly comprising a magnet, the magnetic assembly secured to a threaded element that interfaces with an opposing end of the distraction rod. The system includes a magnetically permeable member in proximity to the magnetic assembly and covering an arc of less that 360° of the adjustable portion.

RELATED APPLICATION

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

FIELD OF THE INVENTION

The field of the invention generally relates to medical devices fortreating disorders of the skeletal system.

BACKGROUND

Scoliosis is a general term for a sideways (lateral) curving of thespine, usually in the thoracic or thoracolumbar region. Often, there isalso a rotation of the spine as well as curvature. Scoliosis is commonlybroken up into different treatment groups. Adolescent IdiopathicScoliosis, Early Onset Scoliosis and Adult Scoliosis.

Adolescent Idiopathic Scoliosis (AIS) typically affects children betweenages 10 and 16, and becomes most severe during growth spurts that occursas the body is developing. One to two percent of children between ages10 and 16 have some amount of scoliosis. Of every 1000 children, two tofive develop curves that are serious enough to require treatment. Thedegree of scoliosis is typically described by the Cobb angle, which isdetermined, usually from x-ray images, by taking the most tiltedvertebrae above and below the apex of the curved portion and measuringthe angle between intersecting lines drawn perpendicular to the top ofthe top vertebrae and the bottom of the bottom. The term idiopathicrefers to the fact that the exact cause of this curvature is unknown.Some have speculated that scoliosis occurs when, during rapid growthphases, the ligamentation flavum of the spine is too tight and hinderssymmetric growth of the spine. For example, as the anterior portion ofthe spine elongates faster than the posterior portion, the thoracicspine begins to straighten, until it curves laterally, often with anaccompanying rotation. In more severe cases, this rotation actuallycreates a noticeable deformity, wherein one shoulder is lower than theother. Currently, many school districts perform external visualassessment of spines, for example in all fifth grade students. For thosestudents in whom an “S” shape or “C” shape is identified, instead an ofan “I” shape, a recommendation is given to have the spine examined by aphysician, and commonly followed-up with periodic spinal x-rays.

Typically, patients with a Cobb angle of 20° or less are not treated,but are continually followed up, often with subsequent x-rays. Patientswith a Cobb angle of 40° or greater are usually recommended for fusionsurgery. It should be noted that many patients do not receive thisspinal assessment, for numerous reasons. Many school districts do notperform this assessment, and many children do not regularly visit aphysician, so often, the curve progresses rapidly and severely. There isa large population of grown adults with untreated scoliosis, in extremecases with a Cobb angle as high as or greater than 90°. Many of theseadults, though, do not have pain associated with this deformity, andlive relatively normal lives, though oftentimes with restricted mobilityand motion. In AIS, the ratio of females to males for curves under 10°is about one to one, however, at angles above 30°, females outnumbermales by as much as eight to one. Fusion surgery can be performed on AISpatients or on adult scoliosis patients. In a typical posterior fusionsurgery, an incision is made down the length of the back and Titanium orstainless steel straightening rods are placed along the curved portion.These rods are typically accrued to the vertebral bodies, for examplewith bone screws, or more specifically pedicle screws, in a manner thatallows the spine to be straightened. Usually, at the section desired,for fusion, the intervertebral disks are removed and bone graft materialis placed to create the fusion. If this is autologous material, the boneis harvested from a hip via a separate incision.

Alternatively, the fusion surgery may be performed anteriorly. A lateraland anterior incision is made for access. Usually, one of the lungs isdeflated in order to allow access to the spine from this anteriorapproach. In a less-invasive version of the anterior procedure, insteadof the single long incision, approximately five incisions, each aboutthree to four cm long are made in several of the intercostal spaces(between the ribs) on one side of the patient. In one version of thisminimally invasive surgery, tethers and bone screws are placed and aresecured to the vertebra on the anterior convex portion of the curve.Currently, clinical trials are being performed which use staples inplace of the tether/screw combination. One advantage of this surgery incomparison with the posterior approach is that the scars from theincisions are not as dramatic, though they are still located in avisible area, when a bathing suit, for example, is worn. The stapleshave had some difficulty in the clinical trials. The staples tend topull out of the bone when a critical stress level is reached.

Commonly, after surgery, the patient will wear a brace for a few monthsas the fusing process occurs. Once the patient reaches spinal maturity,it is difficult to remove the rods and associated hardware in asubsequent surgery, because the fusion of the vertebra usuallyincorporates the rods themselves. Standard practice is to leave thisimplant in for life. With either of these two surgical methods, afterfusion, the patient's spine is now straight, but depending on how manyvertebra were fused, there are often limitations in the degree offlexibility, both in bending and twisting. As these fused patientsmature, the fused section can impart large stresses on the adjacentnon-fused vertebra, and often, other problems including pain can occurin these areas, sometimes necessitating further surgery. Many physiciansare now interested in fusionless surgery for scoliosis, which may beable to eliminate some of the drawbacks of the fusion.

One group of patients in which the spine is especially dynamic is thesubset known as Early Onset Scoliosis (EOS), which typically occurs inchildren before the age of five. This is a more rare condition,occurring in only about one or two out of 10,000 children, but can besevere, sometimes affecting the normal development of organs. Because ofthe fact that the spines of these children will still grow a largeamount after treatment, non-fusion distraction devices known as growingrods and a device known as the VEPTR—Vertical Expandable ProstheticTitanium Rib (“Titanium Rib”) have been developed. These devices aretypically adjusted approximately every six months, to match the child'sgrowth, until the child is at least eight years old, sometimes untilthey are 15 years old. Each adjustment requires a surgical incision toaccess the adjustable portion of the device. Because the patients mayreceive the device at and age as early as six months old, this treatmentrequires a larger number of surgeries. Because of the multiplesurgeries, those patients have a rather high preponderance of infectionand other complications.

Returning to the AIS patients, the treatment methodology for those witha Cobb angle between 20° and 40° is quite controversial. Many physiciansprescribe a brace (for example, the Boston Brace), that the patient mustwear on the body and under their clothes 18 to 23 hours a day until theybecome skeletally mature, for example to age 16. Because these patientsare all passing through their socially demanding adolescent years, it isquite a serious prospect to be forced with the choice of either wearinga somewhat bulky brace that covers most of the upper body, having fusionsurgery that may leave large scars and also limit motion, or doingnothing and running the risk of becoming disfigured and possiblydisabled. It is commonly known that many patients have at times hiddentheir braces, for example, in a bush outside of school, in order toescape any related embarrassment. The patient compliance with bracewearing has been so problematic, that there have been special bracesconstructed which sense the body of the patient, and keep track of theamount of time per day that the brace is worn. Patients have even beenknown to place objects into unworn braces of this type in order to foolthe sensor. Coupled with the inconsistent patient compliance with braceusage, is a feeling by many physicians that braces, even if usedproperly, are not at all effective at curing scoliosis. These physiciansmay agree that bracing can possibly slow down or even temporarily stopcurve (Cobb angle) progression, but they have noted that as soon as thetreatment period ends and the brace is no longer worn, often thescoliosis rapidly progresses, to a Cobb angle even more severe than itwas at the beginning of treatment. Some say the reason for the supposedineffectiveness of the brace is that it works only on a portion of thetorso, and not on the entire spine. Currently a 500 patient clinicaltrial known as BrAIST (Bracing in Adolescent Idiopathic Scoliosis Trial)is enrolling patients, 50% of whom will be treated with the brace and50% of who will simply be watched. The Cobb angle data will be measuredcontinually up until skeletal maturity, or until a Cobb angle of 50° isreached, at which time the patient will likely undergo surgery.

Thought this trial began as a randomized trial, it has since beenchanged to a “preference” trial, wherein the patients choose whichtreatment arm they will be in. This is partially because so manypatients were rejecting the brace. Many physicians feel that the BrAISTtrial will show that braces are completely ineffective. If this is thecase, the quandary about what to do with AIS patients who have a Cobbangle of between 20° and 40° will only become more pronounced. It shouldbe noted that the “20°to 40°” patient population is as much as ten timeslarger than the “40° and greater” patient population.

Currently, genetic scientists have found and continue to find multiplegenes that may predispose scoliosis. Though gene tests have beendeveloped, including a scoliosis score for risk of curve progression,some are still skeptical as to whether gene therapy would be possible toprevent scoliosis. However the existence of a scoliosis gene would nodoubt allow for easier and earlier identification of probably surgicalpatients.

SUMMARY

In one aspect of the invention, a distraction system includes adistraction rod having one end configured for affixation to at a firstlocation on patient. The system further includes an adjustable portionconfigured for placement in the patient at a second location, theadjustable portion comprising a housing containing a magnetic assemblycomprising a magnet, the magnetic assembly secured to a threaded elementthat interfaces with an opposing end of the distraction rod. The systemfurther includes a magnetically permeable member in proximity to themagnetic assembly and covering an arc of less than 360° of theadjustable portion.

In another aspect of the invention, a method for locating a distractionsystem implanted within a patient using a compass having a magnetizedpointer includes placing the compass in proximity to an area of thepatient's skin near an expected location of the magnet of thedistraction system, and observing the direction that a magnetizedpointer of the compass points. The magnetized pointer is then used toconfirm it is pointing to the expected location of the magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the spine of a person with scoliosis.

FIG. 2 illustrates the Cobb angle of a scoliotic spine.

FIG. 3 illustrates the small incisions made during scoliosis non-fusionsurgery of the inventive embodiments.

FIG. 4 illustrates and exemplary distraction device mounted on the spineof the subject.

FIG. 5A is a cross-sectional view of a distraction rod and adjustableportion taken along a perpendicular axis to the longitudinal axis of thedistraction rod.

FIG. 5B illustrates a cross-sectional view of the distraction rod andthe adjustable portion taken along the line B-B′ of FIG. 5A.

FIG. 5C illustrates and enlarged cross-sectional view of detail C ofFIG. 5B.

FIG. 5D illustrates a cross-sectional view of the magnet portion of thedevice, taken along the line D-D′ of FIG. 5C.

FIG. 6 illustrates a distraction device being tested within adistraction loss tester.

FIG. 7A illustrates a perspective view of one end of a distraction rodillustrating the splined tip.

FIG. 7B is a side cross-sectional view of the tubular housing with thelead screw and magnetic assembly removed for clarity.

FIG. 7C is a cross-sectional view of the tubular housing taken along theline C′-C in FIG. 7B.

FIG. 7D illustrates a magnified view of detail D of FIG. 7C.

FIG. 8 illustrates an embodiment of a distraction device having amaintenance member.

FIG. 9 illustrates an embodiment of the distraction device of FIG. 8with a cover sleeve removed and showing maintenance member.

FIG. 10A illustrates the maintenance member of FIGS. 8 and 9.

FIG. 10B illustrates a side view of the maintenance member of FIG. 10A.

FIG. 10C illustrates an end view of the maintenance member of FIG. 10A.

FIG. 11A illustrates the magnetic and mechanical forces acting on acylindrical magnet.

FIG. 11B illustrates the cylindrical magnet being torqued to a finiteamount away from its magnetic orientation with a maintenance member.

FIG. 11C illustrates the cylindrical magnet after being aligned from atorque applied on its north pole by a maintenance member.

FIG. 12 illustrates and external adjustment device that is used with thedistraction devices described herein.

FIG. 13 illustrates a method of using a magnetic compass to locate animplanted magnet with the magnetic compass in an initial location.

FIG. 14 illustrates a method of using a magnetic compass to locate animplanted magnet with the magnetic compass in a final location.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 illustrates a patient 100 with scoliosis. The concave portion 102of the spinal curve can be seen on the left side 104 of the patient 100,and the convex portion 106 can be seen on the right side 108 of thepatient 100. Of course, in other patients, the concave portion 102 mayappear on the right side 108 of the patient 100 while the convex portion106 may be found on the left side 104 of the patient. In addition, asseen in FIG. 1, some rotation of the spine 110 is present, andunevenness between the left shoulder 112 and right shoulder 114 is seen.

FIG. 2 illustrates the Cobb angle 116 of a spine 110 of a patient withscoliosis. To determine the Cobb angle, lines 118 and 120 are drawn fromvertebra 122 and 124, respectively. Intersecting perpendicular lines 126and 128 are drawn by creating 90° angles 130 and 132 from lines 118 and120. The angle 116 created from the crossing of the perpendicular lines126 and 128 is defined as the Cobb angle. In a perfectly straight spine,this angle is 0°.

In many Adolescent Idiopathic Scoliosis (AIS) patients with a Cobb angleof 40° or greater, spinal fusion surgery is typically the first option.Alternatively, non-fusion surgery may be performed, for example with thedistraction device 200 of FIG. 4. FIG. 3 illustrates an upper incision136 and a lower incision 138 formed in the patient 100 which istypically made during non-fusion scoliosis surgery.

FIG. 4 illustrates a distraction device 200 for treating scoliosisaccording to one embodiment of the invention. The distraction device200, which is an implantable device, is fixated at its upper end 202 andlower end 204 to the patient's spine 500. The illustrated example of thespine 500 includes the particular thoracic and lumbar vertebrae thattypically encompass a scoliotic curve, for example the curve of apatient with adolescent idiopathic scoliosis. The T3 through T12thoracic vertebrae, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512,respectively and the L1 through L3 vertebrae, 513, 514, 515 are depictedin FIG. 4, not in a severe scoliotic condition, but in a very slightresidual curve that represents a modest curve that has been partially orcompletely straightened during the implantation procedure.

Each vertebra is different from the other vertebra by its size andshape, with the upper vertebra generally being smaller than the lowervertebra. However, generally, the vertebrae have a similar structure andinclude a vertebral body 516, a spinous process 518, 520, laminae 526,transverse processes 521, 522 and pedicles 524. In this embodiment, thedistraction device 200 includes a distraction rod 206 which isadjustable (lengthwise) via a coupled adjustable portion 208. Thedistraction device 200 also includes a lower short rod 209. Thedistraction device 200 is fixated to the spine 500 via hooks 600, 601 atthe upper end 202 of the distraction rod 206. Alternatively, a clamp maybe secured around an adjacent rib (not shown) of rib facet. In stillanother alternative, a pedicle screw system may be used.

Referring back to FIG. 4, the distraction device 200 is illustrated asbeing fixated to the spine 500 with a pedicle screw system 531, whichattaches directly to the lower short rod 209. The distraction rod 206 isshown after it has been bent into a kyphotic curve, and the lower shortrod is shown after it has been bent into a lordotic curve. As explainedin more detail below. The adjustable portion 208 preferably contains amagnetic assembly having a permanent magnet configured to drive a leadscrew that, depending on the direction of rotation of the internalmagnet, will extend or retract the distraction rod 206 using theadjusting portion 208. Lengthening of the distraction rod 206, forexample, will impart a distraction force to the spine 500. Retractingthe distraction rod 206 will lower or remove the distraction force onthe spine 500, for example if too high a distraction force causes painor complications.

Because a scoliotic spine is also rotated (usually the center section isrotated to the right in AIS patients), the non-fusion embodimentpresented here allows de-rotation of the spine 500 to happen naturally,because there is no fixation at the middle portion of the distractiondevice 200.

In order to further facilitate this de-rotation, the distraction device200 may allow for free rotation at its ends. For example, the adjustmentportion 208 may be coupled to the spine via an articulating joint. U.S.Patent Application Publication Nos. 20090112207 and 20100094302, both ofwhich are incorporated by reference, describe various articulatinginterfaces and joints that may be utilized to couple the adjustableportion 208 to the connecting rods or the like. These Publicationsfurther describe various distraction rod embodiments and methods of usethat may be used with inventions described therein.

As noted, the distraction rod 206 and the lower short rod 209 may bebent by the user (or supplied pre-curved) with the typical shape of anormal saggital spine, but it should also be noted that the curve may beslightly different than standard scoliosis fusion instrumentation,because in the non-fusion embodiment described herein, the distractiondevice 200 is not usually flush with the spine but rather is placedeither subcutaneous or sub-fascial, and thus is not completely below theback muscles. In these less invasive methods, the only portions of thedistraction device 200 that are designed to be placed below the musclesare the hooks 600, 601 and the portion of the distraction rod 206immediately adjacent the hooks 600, 601, the pedicle screw system 531and the lower short rod 209. Thus, FIG. 4 illustrates an embodiment inwhich the bulk of the hardware associated with the distraction device200 is placed over the muscle. It should be understood, however, that inalternative configurations, any other part of the entire implantableembodiment may be placed under the muscle (i.e., sub-muscular). Itshould be appreciated that a much smaller amount of muscle needs to bedissected during the procedure in comparison with current fusionprocedures. This will allow for a much shorter procedure, much lessblood loss, much quicker recovery, and less time in the hospital/lessrisk of infection.

FIGS. 5A-5C illustrate cross-sectional views of the interface of thedistraction rod 206 with the adjustable portion 208. FIG. 5A is across-sectional view of the distraction rod 206 and adjustable portion208 taken along a perpendicular axis to the longitudinal axis of thedistraction rod 206. FIG. 5B illustrates a cross-sectional view of thedistraction rod 206 and the adjustable portion 208 taken along the lineB′-B of FIG. 5A. FIG. 5C illustrates an enlarged cross-sectional view ofdetail C of FIG. 5B. As best seen in FIG. 5C, and end 210 of thedistraction rod 206 includes an elongate recess 212. The elongate recess212 may have a length of around 60 mm. The recess 212 is dimensioned toreceive a lead screw 260. The lead screw 260 may be made from a highstrength material such as, for example, titanium. At least a portion ofthe lead screw 260 includes external threads 262 that are configured toengage with a nut 214 integrated into the recess 212. The nut 214provides a threaded portion on the recess 212 of the distraction rod206. The lead screw 260 may have, for example, 80 threads per inchalthough more or less could be used. The nut 214 may include threads ora chamfered surface 216 on the outer diameter in order to better ensurea secure attachment to the inner diameter of the recess 212 of thedistraction rod 206. For example, the nut 214 may be bonded to thedistraction rod 206 using an adhesive such as EPOTEK 353ND, availablefrom EPOXY TECHNOLOGY, INC., 14 Fortune Drive, Billerica, Mass. Thisallows the distraction rod 206 to be fabricated from a single piece ofstronger material. It also provides for clearance between the lead screw260 and internal diameter of the distraction rod 206. Alternatively, athreaded portion may be directly formed in the recess 212 without theaid of a separate nut 214. A radially-poled cylindrical magnet 254 ispart of a magnetic assembly 236 comprising a first cup 240 and a secondcup 242. The first and second cups 240, 242 are made from titanium. Thisentire magnetic assembly 236 is attached to the lead screw 260, forexample by a high strength pin 238 which is placed through a hole in thelead screw 260 and a receptacle 244 in the first cup 240. This couplesthe cylindrical magnet 254 to the lead screw 260. The cylindrical magnet254 typically has two poles, a North and a South that are radiallyarrayed, as depicted in FIG. 5D. The cylindrical magnet may comprise arare earth material, such as Neodymium-Iron-Born. The cylindrical magnet254 is attached to a thrust bearing 250 and a radial bearing 245, whichallow the low friction rotation of the cylindrical magnet 254, and thisaids the low friction rotation of the lead screw 260 within the nut 214.This allows for the non-invasive coupling of an external moving magneticfield, in order to non-invasively distract the distraction device 200,allowing the distraction rod 206 to telescopically extend from theadjustable portion 208, and impact an increased distraction force on thespine 500. The moving magnetic field may be supplied by one or morerotating magnets, for example as part of a motor driven external device.Alternatively, the moving magnetic field may be produced by anelectromagnetic coil. The lead screw 260 and nut 214 combination allowsfor a device that can be distracted or retracted. The device isretracted by making the external moving magnetic field move in theopposite rotational direction. This is an advantage, for example in thecase of a patient that has accidentally been over distracted. Thedistraction device 200 may then be retracted somewhat, until the patientis at the preferred distraction amount. An elastomeric o-ring 234creates a dynamic seal between the inner surface of the adjustableportion 208 and the distraction rod 206. This o-ring resides inside arecess 232 of an o-ring gland 230 within the interior of the adjustableportion 208.

The low friction lead screw 260 and nut 214 combination combined withthe low friction bearings 250, 246 minimize the torque that needs to beapplied on the cylindrical magnet 254. Thus, they also minimize therequired size of the cylindrical magnet 254, because they minimize themagnetic force required to make the cylindrical magnet 254 turn.However, these same advantages also may make the assembly prone to losesome of the distraction length as the patient moves through dailyactivity. For example (returning to FIG. 4), it may be possible for apatient's movement to create a “screw-like” motion which is capable ofslowly retracting the distraction rod 206 in relation to the adjustableportion 208, and thus shortening the distraction device 200 by multiplesof very small movements. For example, in the process of walking,running, bending or other movements, a patient may place a compressivebending force (F) on the distraction device 200. In those movements, thepatient may also place a torque (T) between the two ends of thedistraction device 200, for example, the two ends at the portions thatare secured to the spine 500. In FIG. 4, a positive value of torque (T)denotes a right-hand mode, in which the distraction rod 206 is givenenergy to move in the direction of the arrow at torque (T) while theadjustable portion is given energy to move in the oppositecircumferential direction. A negative value of torque (T) wouldrepresent the opposite, left hand motion. If there are no internalfeatures in the distraction device 200 to limit the circumferentialmotion of the distraction rod 206 in relation to the adjustable portion208, the a positive value of torque (T) will cause the distraction rod206 and adjustable portion 208 to circumferentially displace until, forexample, the torsional movement in the patient stops, either willingly,or by the physical limitations in the spine or the rest of body. If thepatient's movements cycle between bending and twisting, and therefore,between the force (F) and torque (T) depicted, they may do so in such away was to cause a multiplicity of slight angular turns of the leadscrew 260 in one direction in relation to the nut 214, withoutcompensatory turns in the opposite direction. For example, referring toFIG. 6, in laboratory testing, a distraction device 200 was secured withset screws 217, 219 in a distraction loss tester 211 having simulatedvertebrae 213, 215 in order to place controlled axial compressive force(F) and a controlled twisting torque (T) on the distraction device 200.One cycle of the program consisted of a 100 Newton compressive force(F), followed by a 0.81 Newton-meter torque (T), after which thecompressive force (F) was completely released (0 Newton) and then anopposite torque (−0.81 Newton-meter) (−T) was placed. These parametersare considered extreme in relation to a typical patient's movements, butare effective in estimating “worst-case” operation, for example, if thedistraction device 200 were being used as a single device within a veryactive patient. A distracted distraction device 200 tested under theseparameters was able to lose several mm of distraction length after about10,000 cycles, which is estimated be the equivalent of about one week ina patient (though actual patient movements are usually much morevariable).

In reality, the preferred design for a distraction device 200, does notallow significant circumferential motion between the distraction rod 206and the adjustable portion 208. FIG. 7A illustrates a perspective viewof the splined tip 220 of the distraction rod 206. The splined tip 20 isillustrated with four (4) protrusions 222 that interface with four (4)corresponding longitudinal grooves 224 (two parts in symmetricopposition) formed inside a tubular housing 226 (illustrated by FIGS.7B-D) of adjustable portion 208. The longitudinal grooves 224 may beformed by wire EDM machining or by broaching. While FIGS. 7A-7Dillustrate an embodiment that uses four (4) protrusions 222 along withfour (4) longitudinal grooves 224 there may be more or fewer. The tighttolerance of the splinted tip 220 with the longitudinal grooves 224keeps the distraction rod 206 centered within the tubular housing 226.In addition, the combination of the splined tip 220 and correspondinggrooves 224 act as an anti-rotation feature that prevents thedistraction rod 206 from rotating relative to the tubular housing 226.This may be necessary to allow the distraction device 200 to be“rigidized” in the even that the device is used in fusion applications,instead of the non-fusion applications described. For example, in afusion application, it is desired that the spine 500 not be able to flexor rotate much during the months that the fusion is taking place. Ineither the fusion applications or the non-fusion applications, theanti-rotation features are intended to limit inadvertent extensionand/or retraction of the distance rod 206 resulting from, for instance,patient movements.

FIG. 7C is a cross-sectional view of the tubular housing 226 taken alongthe line C′-C in FIG. 7B. FIG. 7D illustrates a magnified view of detailD of FIG. 7C. In this illustrated embodiment, as best seen in thedetailed view of FIG. 7D, small reliefs 228 are incorporated into thesides or corners of the longitudinal grooves 224. These reliefs 228 maybe slight over cut wire EDM notches that prevent the corners of theprotrusions 222 from contacting the inner wall of the tubular housing226. Less contact between the protrusions 222 and the longitudinalgrooves 224 results in less frictional forces and reduces the likelihoodof binding. Optionally, the tops of the protrusions 222 could be curved,for example, cut from a diameter instead of a square. This rounding ofthe protrusions 222 would keep the protrusions 222 from binding with thelongitudinal grooves 224 when torsional stresses are imparted betweenthe distraction rod 206 and the adjustable portion 208. This optionalmodification makes the distraction rod 206 easier to manufacture andeliminates the need for the relief 228 overcuts. At the maximum amountof axial distraction length, the protrusions 222 butt up against a stop231 (as seen in FIG. 5C), so that the distraction rod 206 terminates itsaxial movement in relation to the adjustable portion 208.

The anti-rotation features of FIGS. 7A-7D are effective in severelyminimizing distraction loss in a large variety of patient applications,however, under severe conditions, such as those described in FIG. 6, adistraction device 200 with these features may still lose as much as 1mm over 10,000 cycles. An additional design improvement which takesadvantage of the magnetic poles (FIG. 5D) of cylindrical magnet 254 willnow be described, as a way to severely limit distraction loss, even inthe most severe performance conditions.

FIG. 8 is a view of a distraction device 300 which does not allowdistraction loss when subjected to the severe testing parametersdescribed above. Distraction device 300 has distraction rod 306 andadjustable portion 308, and the device is identical to the distractiondevice 200 described in the prior figures, except that it also comprisesa maintenance member 303 as seen in FIG. 9. A lower short rod 309extends from the adjustable portion 308 in a direction opposite thedistraction rod 306. FIG. 9 shows the maintenance member 303 withexternal cover 305 removed. FIG. 8 shows the completed device with themaintenance member 303 completely covered by external cover 305. Theexternal cover 305 is made of Titanium or Titanium alloy, for example,and is welded to the exterior of the adjustable portion 308 (alsoTitanium or titanium alloy) to completely isolate maintenance member 303from the patient. The maintenance member 303 is made from a magneticallypermeable material such as iron or mu-metal (75% nickel, 15% iron, pluscopper and molybdenum). The maintenance member may also be made from abiocompatible and typical implant material such as 400 series stainlesssteel, for example 420 stainless steel. Alternatively to being isolatedwithin the double wall of the adjustable portion 308 of the distractiondevice 300 as depicted, a portion of the outer wall of the adjustableportion 308 may be made from a magnetically permeable material, such as400 series stainless steel and the remainder of the outer wall may bemade of a material like Titanium or Titanium alloy, without significantmagnetic properties. The maintenance member 303 may also be coated witha biocompatible material. As illustrated in FIGS. 10A, 10B and 10C, themaintenance member 303 has an arcuate shape with an arc of less than360°. In some embodiments, the arc may be less than 180° or 120°. In theembodiment of FIGS. 10A, 10B and 10C, the arc (A) is approximately 99°although a smaller arc may be used. The length (L) of the maintenancemember 303 is 19 mm in the embodiment depicted. In the adjustableportion 308 of the distraction device 300, the maintenance member 303 islocated axially in the same portion as the cylindrical magnet 254.Because of its magnetically permeable characteristics, the maintenancemember 303 will most strongly attract the north pole or the south poleof the cylindrical magnet 254, but will not attract the portion which ishalfway between the north pole and south pole. Referring to FIG. 10C,the maintenance member 303 preferably has a thickness (t) of at least0.2 mm, and more preferably at least 0.3 mm. A typical thickness of themaintenance member 303 is 0.48 mm (0.019 inches) or less. In theconfiguration depicted, the radius of curvature of the concave surfaceof the maintenance member 303 is 4.5 mm but this thickness could beless. The diameter of the adjustable portion 308 is 10.5 mm over theexternal cover 305, and is 9.0 mm over the non-magnetic portion 311.

FIGS. 11A, 11B and 11C demonstrate the effect of the maintenance member303 in maintaining the circumferential orientation of the cylindricalmagnet 254, and thus the circumferential orientation of the lead screw260 (FIG. 5C), and thus the amount of axial distraction in thedistraction device 300. In FIG. 11A, the centerpoint 315 of the northpole 319 of the cylindrical magnet 254 aligns with the center of mass313 of the maintenance member 303. When patient movement causes a firsttorque T1 to be applied to the cylindrical magnet 254, a magneticcorrective torque T2 based on the magnetic field 317 between themaintenance member 303 and the cylindrical magnet 254 acts upon thecylindrical magnet 254. If the first torque T1 is less than thecorrective torque T2, then the magnet will not be displaced. It shouldbe noted however that the corrective torque T2 increases as thedisplacement angle α increases. When a significantly large torque isplaced on the cylindrical magnet 254 (FIG. 11B), for example incombination with a significant comprehensive force on the distractiondevice 300, the lead screw will slightly turn, and thus the magnet willslightly turn. Because the maintenance member 303 is present, themagnetic field 317 will return the cylindrical magnet 254 to itsoriginal circumferential orientation (FIG. 11C), for example when thecompressive force on the distraction device 300 is released.

It should be noted that the distraction force which can be achieved inthe distraction device 300 will be somewhat less with the maintenancemember 303 in place without it. For example, a distraction device 300that achieves a distraction force of 220 Newtons without the maintenancemember 303 will achieve a distraction force of about 195 Newtons (12%reduction) with the maintenance member 303 in place. The circumferentialorientation of the distraction device 300, and thus the circumferentialorientation of the maintenance member 303 within the body, does notcause a large difference in the ability to distract the distractiondevice 300, because typically very large, overpowering magnets are usedexternally to distract the implanted distraction device 300. Typically,two such magnets are located in an external adjustment device and arerotated to impart rotational motion to the cylindrical magnet 254.Particular details on the nature of the external adjustment devices thatcan be used in connection with the distraction devices described hereinare disclosed, for example, in U.S. Patent Application Publication Nos.2009/0112207, 2010/0094302, 2010/0121323, and U.S. patent applicationSer. No. 13/172,598, all of which are incorporated by reference herein.FIG. 12 illustrates an external adjustment device 400 according to oneembodiment that includes two permanent magnets 402, 404 contained withinrespective covers 406. Each permanent magnet 402, 404 is rotatablewithin its respective cover 405 and provides a moving magnetic field. Amotor 408 is mechanically engaged to the permanent magnets 402, 404 viaa transmission (not shown) contained within a housing 410 of theexternal adjustment device 400.

Distraction devices constructed with the maintenance member 303 asdescribed, having a cylindrical magnet 254 diameter of less than 9 mm,have been rested in the severe 100 Newton, comprehensive force/0.81Newton-member torque cyclic regime as described, and have maintainedtheir distraction amount of over hundreds of thousands of cycles. Thesedevices have also been implanted in patients who are physically active,and the implanted devices have maintained their distraction amount overseveral months of patient activity.

Another advantage of the maintenance member 303 is that, whendistracting the distraction device, the device is adjusted to veryspecific gradations. For example two per rotation of the magnet, wheneither the south pole or the north pole aligns with the center of mass313 of the maintenance member 303. For example, these graduations may beequal to approximately 0.16 mm in the present case, or even morepreferably 0.10 mm or 0.20 mm.

In one alternative embodiment, there are two or more magnetic members303, for example, one magnetic member matched with each of the poles ofthe cylindrical magnet. In another alternative embodiment, thecylindrical magnet may be replaced by two or more magnetically permeablecomponents, each spanning less than 180°of an arc, and the maintenancemember may be replaced with a magnet. The maintenance member may includea number of configurations including a plate, a wire, a series of balls,or any other configuration of magnetically permeable material.

FIGS. 13 and 14 describe a method for locating an implanted magnet 702within a patient 700. This can be used for any magnetic medical device,but will be described specifically for a distraction system containing amagnet. The method utilizes standard magnetic compasses, which can beobtained very easily all over the world. The method consists ofproviding a magnetic compass 704, providing instructions for a user forusing the magnetic compass 704 to locate an implanted magnet 702 withinthe distraction system, wherein the instructions instruct the user toplace the magnetic compass 704 in proximity to a general area 708 of thepatient's skin near the expected location of the implanted magnet 702 ofthe distraction system, and for the user to view the direction that amagnetized pointer 706 of a needle 705 of the magnetic compass 704points, and for the user to determine that the magnetized pointer 706 ispointing towards the implanted magnet 702, wherein the instructionsfurther instruct the user to move the magnetic compass 704 along theskin of the patient until the magnetized pointer 706 is perpendicular tothe skin (FIG. 14), and for the user to determine that the location 710to which the magnetic compass 704 is moved is the correct location forplacing an external adjustment device 400 which is enabled to cause thedistraction system to magnetically distract.

As illustrated in FIGS. 13 and 14, the patient 700 has an implantedmagnet 702 which is implanted near the lower back area, and the patientlies prone while the magnetic compass 704 is held to the side of thepatient, with the needle 705 able to spin on the horizontal plane, andwith the magnetic compass 704 held at approximately the same horizontalplane as the depth of implantation of the implanted magnet 702. In FIG.13, the magnetic compass 704 is initially held further distal (towardsthe feet) than the implanted magnet 702, with the magnetized pointer 706of the needle 705 pointing at an angle towards the implanted magnet 702.The magnetic compass 704 is moved in direction (D) until it is locatedas in FIG. 14, with the magnetized pointer 706 of the needle 705perpendicular to the skin. Using the method described, small implantedmagnets may be successfully located. For example, the location of amagnet having a mass of 5.4 grams can be correctly identified with astandard low-cost magnetic compass at an implant depth of as much as 10cm.

While embodiments have been shown and described, various modificationsmay be made without departing from the scope of the inventive conceptsdisclosed herein. The invention(s), therefore, should not be limited,except to the following claims, and their equivalents.

What is claimed is:
 1. A distraction system comprising: a distractionrod having one end configured for affixation to at a first location onpatient; an adjustable portion configured for placement in the patientat a second location, the adjustable portion comprising a housingcontaining a magnetic assembly comprising a magnet, the magneticassembly secured to a threaded element that interfaces with an opposingend of the distraction rod; and a magnetically permeable member inproximity to the magnetic assembly and covering an arc of less than 360°of the adjustable portion.
 2. The distraction system of claim 1, whereinthe magnetic permeable member covers an arc of less than 180° of theadjustable portion.
 3. The distraction system of claim 1, wherein themagnetic permeable member covers an arc of less than 120° of theadjustable portion.
 4. The distraction system of claim 1, wherein themagnetic permeable member covers an arc of approximately 99° of theadjustable portion.
 5. The distraction system of claim 1, wherein themagnetic permeable member comprises iron.
 6. The distraction system ofclaim 1, wherein the magnetic permeable member comprises mu-metal. 7.The distraction system of claim 1, wherein the magnetic permeable membercomprises stainless steel.
 8. The distraction system of claim 1, whereinthe magnetic permeable member comprises 400 series stainless steel. 9.The distraction system of claim 8, wherein the magnetically permeablemember comprises 420 stainless steel.
 10. The distraction system ofclaim 1, wherein the magnetically permeable member is enclosed within anon-ferrous metal.
 11. The distraction system of claim 10, wherein themagnetically permeable member is enclosed within Titanium or an alloy ofTitanium.
 12. The distraction system of claim 1, wherein themagnetically permeable member comprises a ferrous material.
 13. Thedistraction system of claim 12, wherein the magnetically permeablemember is coated with a biocompatible material.
 14. The distractionsystem of claim 1, wherein the magnet is a cylindrical magnet.
 15. Thedistraction system of claim 1, wherein the magnet has two poles.
 16. Thedistraction system of claim 14, wherein the magnet has two radiallyarrayed poles.
 17. The distraction system of claim 1, wherein the magnetis a rare earth magnet.
 18. The distraction system of claim 17, whereinthe magnet comprises Neodymium-Iron-Boron.
 19. The distraction system ofclaim 1, wherein the distraction system, when subjected to 10,000 cyclesof a 100 Newton compressive force, followed by a 0.81Newton-meter-torque, followed by release of the compressive force to 0Newtons and then an opposite torque of −0.81 Newton-meter, will loseless than 0.1 mm of distraction length.
 20. The distraction system ofclaim 1, wherein the diameter of all portions of the distraction systemis 10.5 mm or less.